In recent years, a light scanning device for making scanning with using an oscillating mirror which oscillates rotationally in reciprocating manner by a sinusoidally oscillating element such as a galvanometer has been proposed, in stead of a light scanning device for making scanning by deflecting a light beam with a polygon mirror that rotates at a constant speed. And also, an image forming apparatus which electro-photographically forms an image with using such light scanning device has been proposed. Since the light scanning device of this kind can scan the light beam reciprocally, it is considered that a detection unit such as a BD sensor for determining a write timing for one way and a detection unit such as a BD sensor for determining a write timing for the other way are provided and the image is written by scanning in both ways.
However, if a plurality of detection unit such as BD sensors is provided, the manufacturing costs increase. Thus, it has been proposed that if one BD sensor detects a light beam to generate a BD signal, a counter is activated to generate a pseudo BD signal instead of another BD sensor (e.g., refer to JP-A-2004-230629).
However, in generating the pseudo BD signal using the counter as described above, a process or a circuit configuration becomes complex and the sufficient accuracy is not attained. Thus, it has been proposed that two stationary mirrors for reflecting the scanned light beam to the BD sensor are provided, whereby one BD sensor detects the timings for both ways in FIG. 9 of JP-A-2004-230629. However, even if the BD sensor detects the light beam in this case, it can not be determined which one of the stationary mirrors the light beam is reflected. Therefore, in a case where two stationary mirrors are provided as shown in FIG. 9 of JP-A-2004-230629, there is possibility that the image to be written during scanning for one way is written during scanning in the other way, to form a laterally reversed image.